Due to rapidly increasing traffic in optical communications in recent years, the transmission capacity of presently used single-core optical fibers is approaching its limit. So, as a means to further expand the communication capacity, multicore fibers in which a plurality of cores are formed in one fiber have been proposed. Use of multi-core fibers can suppress the laying cost of optical fibers and expand the transmission capacity.
When a multi-core fiber is used as a transmission path, each of the core parts of the multicore fiber needs to be connected to another multi-core fiber, an optical element, or the like respectively to send and/or receive transmission signals. An optical fiber ribbon, in particular, including a plurality of such multi-core fibers arranged in parallel can easily connect a plurality of multi-core fibers collectively.
On the other hand, a multi-core fiber has cores arranged at positions other than the center of the cross section, and this leads to a problem that the connection thereof is more difficult compared to connection of single-core fibers.
Thus, for an optical fiber ribbon having a plurality of such multi-core fibers arranged in parallel, it is preferable that the arrangement of the cores of each of the multi-core fibers is aligned in a particular direction.
As a method for specifying the core arrangement in a multi-core fiber, Japanese Unexamined Patent Application Publication No. 2011-170099 (JP-A-2011-170099) discloses a multi-core fiber provided with a marker on a cladding thereof for identifying core arrangements.
In addition, it has been expected to reduce connection loss in connecting optical fiber ribbons together or in connecting an optical fiber ribbon with a light element or the like. To reduce the connection loss, it is required to further reduce shifting of axes of cores, and thus it is preferable that optical fibers in an optical fiber ribbon are aligned so that the eccentric direction of cores of each of the optical fibers is in one fixed direction.
However, even with the optical fibers disclosed in Patent Document 1 in which the core arrangement is identifiable on its cross section, it is still difficult to form an optical fiber ribbon by aligning the optical fibers with cores of each of the optical fibers arranged in a particular direction.